1. Field of the Invention
The present invention relates to AV (Audio Video) devices connected to a network handling Audio and Video signals, such as typified by digital interfaces conformable to IEEE (the Institute of Electrical and Electronics Engineers, inc.) 1394 standard, and to a method of controlling the AV devices.
2. Description of the Related Art
As recently known, by using a serial bus with a digital interface compliant with IEEE 1394 standard, it is possible to build up a network system in which a plurality of AV devices are connected in an arbitrary manner.
In particular, the above IEEE 1394 serial bus is used as a digital interface between digital broadcasting receivers such as an STB (Set Top Box) and digital recording/reproducing devices such as D-VHS (Digital-Video Home System) systems. This serves to build up a high definition recording/reproducing system.
When carrying out digital recording from an STB to a D-VHS device using the above IEEE 1394 serial bus, connection is made in the following manner.
(1) The STB secures isochronous CH (Channel) with respect to IRM (Isochronous Resource Manager).
(2) The STB secures isochronous bandwidth with respect to the IRM.
(3) Logical connection (new connection) between oPCR (output Plug Control Register, output logical plug) of the STB and iPCR (output Plug Control Register, input logical plug) of the D-VHS device.
(4) The STB starts isochronous transmission of MPEG [Moving Picture Experts Group] (partial TS: Partial Transport Stream) from the STB. By doing so, MPEG data is transmitted to the D-VHS device.
(5) The STB requests AV (Audio Video)/C (Control) command to the D-VHS device.
(6) The D-VHS device starts recording while making logical connection (Overlay connection) between the oPCR of the STB and the iPCR of the D-VHS device.
The overlay connection is not essential process. By contrast, the logical connection cannot be opened by any devices other than one that has achieved it. When the device opens a logical connection, a D-VHS digital recording/reproducing device, for example, which is operating in association with this device, fails to perform stable data recording. To prevent this, the D-VHS device achieves logical connection as an overlay connection.
By doing so, when recording by the D-VHS device is carried out, the logical connection from the D-VHS device still remains even if the connection with the partner device is opened; therefore, stable isochronous transmission can continue, and as a result, the recording operation can be stably continued.
After steps (1) to (6) are carried out, the user may designate only a desired part of the program. Then, steps (1) to (6) will be performed in the reverse order. Namely, the following steps will be performed in sequence:
(1) The STB requests an AV/C command (stop operation) with respect to the D-VHS device.
(2) The D-VHS device completes recording while opening a logical connection (Overlay connection) between the oPCR of the STB and the iPCR of the D-VHS device.
(3) The STB stops isochronous transmission of MPEG (partial TS).
(4) The logical connection between the oPCR of the STB and the iPCR of the D-VHS device is opened or eliminated. The next process is carried out.
(5) The STB returns the isochronous bandwidth to the IRM.
(6) The STB returns the isochronous CH to the IRM.
During the recording operation, when users make the operations such as device expansion (addition) and separation (deletion) from the bus, bus reset occurs in the IEEE 1394 serial bus. The bus reset is a standardized function for detecting a change (i.e., change of connection tree structure, herein) of network topology in IEEE 1394 and transmitting it to devices on the network.
When the above bus reset occurs, it is possible to acquire node information of a new bus within a specified time, that is, time (about one second) defined by isoch_resource_delay, and to continue isochronous transmission.
Namely, the connection is not slowly restored over a very long time. For example, if two networks are connected via cable, one network is newly built up. However, when two networks before being connected use the same logical channel independently from each other, transmission data collides after connection.
If the above-mentioned disadvantage is continued, the problem arises. For this reason, there is a need for limiting the connection restore so that it can be completed within a predetermined time. Alternatively, there is a need for stopping transmission once and restarting the transmission after a new logical channel is secured.
For data, such as AV stream, which should be continuously transmitted, the connection should be restored within a predetermined time. In IEEE 1394 networks, the predetermined time is defined as isoch_resource_delay.
According to the above connection restore, there is no assurance that isochronous transmission is always continuously secured without being disconnected. For this reason, the instruction manual for a device states the following warning message: “Do not plug in and out cable during recording even if the device is an unnecessary device.”
However, the IEEE 1394 serial bus makes high-speed digital communication from 100 Mbps (Mega bits per second) to 400 Mbps; for this reason, it is inevitably influenced by external noise. As a result, bus reset occurs even if users do not intentionally expand or reduce the number of devices on the network. Thus, it is difficult to fully eliminate the influence of noise, and to specify the noise source by user.
After the bus reset occurs, unless connection is restored within the time defined by the above isoch_resource_delay, that is, if reconnection fails, the connection relationship on a network made so far becomes invalid, and a new network is built up.
For this reason, the following problem arises. In the above procedure, that is, in a state that the STB carries out digital recording with respect to the D-VHS device, bus reset occurs and reconnection fails. In this case, the STB cannot carry out digital recording with respect to the D-VHS device until reset is made on a new network.
There are some reasons why the reconnection cannot be achieved. First, the node information is not acquired within a limited time because the time required to acquire the information differs according to the type of the device or the timing of data acquisition. Second, two or more devices request for the IRM resource at the same time.
As described above, under the present circumstances, when bus reset occurs during a recording operation and reconnection fails, a problem arises such that recording is interrupted.
JPN. PAT. APPLN. KOKAI Publication No. 11-355329 discloses the following technique. According to the technique, if a relatively lengthy bus-reset occurs, data read or write to a recording medium is temporarily stopped while synchronous packet transmission is interrupted. By doing so, it is possible to prevent transmission data error and the generation of a non-recording portion.
JPN. PAT. APPLN. KOKAI Publication No. 2000-124928 discloses the following technique. According to the technique, several information transmission systems transmitting information under the IEEE 1394 standard are mutually connected, and thereby, a new information transmission system is formed. Even if bus reset occurs in the entirety of the new information transmission system, information transfer is normally carried out between at least two nodes after the bus reset.
However, the above KOKAI Publications have no description relative to suitable steps for interruption of information transmission when bus-reset occurs during a recording operation and reconnection fails, as described above.